1. Field of the Invention
The present invention generally relates to data reproduction apparatuses for reproducing recorded data from media such as optical disks, and more particularly to a data reproduction apparatus for reproducing data from a reproduced signal from a recording medium recorded with data in accordance with a recording signal of a partial response waveform.
2. Description of the Related Art
For instance, attempts have been made to use a magneto-optical disk unit, which is a data reproduction apparatus, in a variety of fields such as the recording and reproduction of image information and various code data for computers because of its large capacity, low price, and high reliability. Such a magneto-optical disk unit is expected to have a larger capacity, and is required to record data with high density and reproduce the data with high accuracy.
Proposed as a method of recording and/or reproducing data with high accuracy is a technique to detect maximum likelihood (ML) data by using a maximum likelihood data detector such as a Viterbi detector after sampling at a given sampling frequency a reproduced signal from a magneto-optical disk recorded with a recording signal obtained by modulating data to be recorded into a so-called partial response (PR) waveform.
Such a Viterbi detector has a structure shown in FIG. 1.
In FIG. 1, a Viterbi detector 100 includes a branch metric calculation unit (hereinafter referred to simply as a BM) 101, an Add-Compare-Select unit (hereinafter referred to simply as an ACS) 102, a path metric memory (hereinafter referred to simply as a PMM) 103, and a path memory (hereinafter referred to simply as a PM) 104.
In the Viterbi detector 100 that is applied to the data reproduction system of a magneto-optical disk unit, a sampled value yt of a reproduced signal from the magneto-optical disk is supplied to the BM 101, and the BM 101 calculates a branch metric value (hereinafter referred to simply as a BM value) that is a difference between the sampled value yt and each expected value. The expected values, which depend on a partial response waveform used in data recording, are values that the reproduced signal is expected to take correctly. The BM value is calculated with respect to each expected value when the one sampled value yt is supplied to the BM 101.
The ACS 102 adds the BM values to path metric values (hereinafter referred to simply as PM values) of a preceding clock timing, and compares each given two of obtained PM values. Then, as a result of the comparisons, the ACS 102 selects the smaller of each given two PM values as a new PM value, and stored the selected PM values in the PMM 103. As a result of such a process, BM values are accumulated to be the PM values. Selecting the smaller of each given two PM values as previously described corresponds to selecting a state transition path. That is, the ACS 102 always selects a state transition path that minimizes the PM value.
Data (binary data) corresponding to the paths selected in the above-described manner is supplied from the ACS 102 to the PM 104. The PM 104 shifts the data corresponding to the selected paths successively, and in the process, discards data corresponding to paths determined unselected based on the continuity of state transition. Then, the PM 104 outputs data corresponding to a survivor path as detected data.
As previously described, by detecting the maximum likelihood data by using the Viterbi detector 100 as well as modulating recording data into a recording signal having a partial response waveform and recording the recording signal on the magneto-optical disk, data recorded with high density on the magneto-optical disk is reproduced therefrom with high accuracy. Such a recording and reproduction technique is called a partial response maximum likelihood (hereinafter referred to simply as PRML) technique.
For instance, in the case of recording, in accordance with a PR(1, 1) waveform, a magneto-optical disk with a signal obtained by modulating data of the 1T sequential pattern xe2x80x9c10101010 . . . xe2x80x9d, sampled values of a signal reproduced from the magneto-optical disk are as shown in FIG. 2. That is, since a recording signal based on PR(1, 1) corresponds to a bit string obtained by adding the bit string of the data and the bit string of the data shifted by one bit (1+D), the sampled values of the reproduced signal corresponding to the recording signal are xe2x80x9c00001111110000000xe2x80x9d as shown in FIG. 2 when the data is xe2x80x9c00001010100000000xe2x80x9d.
In the case of inputting the sampled values shown in FIG. 2 to the Viterbi detector 100 of FIG. 1, the paths selected in the ACS 102 and states has a relation shown in FIG. 3.
In FIG. 3, the states are indicated by circles and the state transition paths are indicated by arrows. In this case, each state is either 0 or 1, and the state transition paths are classified into four types: state 0 to state 0, 0 to 1, 1 to 0, and 1 to 1. A solid arrow indicates a path to state 1, and a dotted arrow indicates a path to state 0.
In the PM 104, the paths selected in the ACS 102 are discarded one after another so that a survivor path is determined. For instance, in (2) through (9) of a time series shown in FIG. 3, neither solid nor broken arrow can be defined as a path (no-merge), and, thereafter in the time series, if such state is entered that paths can be determined (merge), either solid or broken arrows are defined as paths. As a result, for instance, arrows enclosed by a dotted line in FIG. 4 become the survivor path, and data xe2x80x9c0000101010000000xe2x80x9d corresponding to the arrows enclosed by the dotted line is obtained as detected data, which matches the original data.
In some cases, the sampled values shown in FIG. 2 are deviated by the effect of noise. Suppose that the reproduced signal (sampled values) that should be obtained correctly as shown in FIG. 2 is obtained as shown in FIG. 5 in which the level of the reproduced signal is lowered by a noise at a time (9). In this case, if sampled values shown in FIG. 5 are input to the Viterbi detector 100 one after another, the paths selected in the ACS 102 and states has a relation shown in FIG. 6. In FIG. 6, only a path at the time (9) is different from that in FIG. 3. However, detected data includes more than one error bit.
If the sampled values shown in FIG. 5 are input to the Viterbi detector 100, paths enclosed by a dotted line in FIG. 7 are defined as a survivor path in the PM 104. Paths in (3) through (9) of the time series in FIG. 7 are different from paths corresponding to the data shown in FIG. 4 which data should correctly be detected. That is, the above-described error path (at the time (9)) causes the detected data to include six error bits.
As described above, since the conventional Viterbi detector determines paths based on the continuity of state transition, a determined (survivor) path may includes sequential errors if a sampled value has an error to change a corresponding path. As a result, detected value corresponding to the determined path may include sequential bit errors.
It is an object of the present invention to provide a data reproduction apparatus having a high data detection capability and good noise immunity.
The above object of the present invention is achieved by a data reproduction apparatus claimed in claim 1 sampling at a given rate a reproduced signal complying with a partial response waveform, calculating branch metric values in accordance with the Viterbi decoding algorithm by employing expected values defined by the partial response waveform and each of sampled values, calculating path metric values based on the branch metric values, selecting paths corresponding to data state transition in accordance with a given rule based on comparison results of the path metric values, and reproducing data based on the selected paths, the reproduced signal being obtained from a recording medium, which data reproduction apparatus includes difference value calculation means for calculating a difference between compared ones of the path metric values in selecting a corresponding one of the paths, alternative path selection means for selecting a path different from the corresponding one of the paths, which are selected in accordance with the given rule, as an alternative path based on the difference between the compared ones of the path metric values calculated in the difference value calculation means, and reproduced data determination means for determining data to be reproduced based on the paths selected in accordance with the given rule and the alternative path selected in the alternative path selection means.
According to the above-described data reproduction apparatus, it can be considered that greater the difference between the compared ones of the path metric values in selecting the corresponding one of the paths, higher the reliability of the selected path, while smaller the difference (closer the compared ones of the path metric values), lower the reliability of the selected path. The data to be reproduced is determined by considering the paths selected in accordance with the given rule and the alternative path selected based on the difference between the compared ones of the path metric values which difference is capable of representing the reliability of the corresponding one of the selected path.
If the reproduced signal obtained from the recording medium complies with the partial response waveform, it does not matter whether a recording signal recorded on the recording medium itself complies with the partial response waveform. Data may be recorded on the recording medium in accordance with a recording signal of the partial response waveform, or, for instance, the reproduced signal may be obtained in a state where waveform interference like the partial response waveform is substantially caused by optically reading a plurality of recording pits collectively in a reproduction system.
As claimed in claim 2, from a relation between the difference between the compared ones of the path metric values and the reliability of the selected path, the alternative path selection means may include judgment means for judging whether the difference between the compared ones of the path metric values is smaller than a predetermined value, and the alternative path may be selected if the judgment means judges that the difference between the compared ones of the path metric values is smaller than the predetermined value.
As a condition for selecting the alternative path, other conditions may be provided based on the difference of the compared ones of the path metric values. For instance, the alternative path is determined based on the priorities of the differences between the compared ones of the path metric values, which priorities are given in descending order of their magnitudes.
As claimed in claim 3, in the above-described data reproduction apparatus, the reproduced data determination means may include: first candidate data generation means for generating a first candidate data string corresponding to a first path string selected in accordance with the given rule; second candidate data generation means for generating a second candidate data string corresponding to a second path string obtained by changing a path of the first path string, the path corresponding to the alternative path selected in the alternative path selection means; and data selection means for selecting one of the first and second candidate data strings based on a given criterion for likelihood, and the first or second candidate data string selected in the data selection means may be determined as the data to be reproduced.
As claimed in claim 4, in the light of determining likely data on the basis of an actual sampled value string of the reproduced signal, in the above-described data reproduction apparatus, the data selection means may include: first sampled value generation means for generating a first candidate sampled value string from the first candidate data string based on a characteristic of the partial response waveform; second sampled value generation means for generating a second candidate sampled value string from the second candidate data string based on the characteristic of the partial response waveform; and actual sampled value judgment means for judging which of the first or second candidate sampled value string is closer to a sampled value string obtained from the reproduced signal, and one of the first and second candidate data strings may be selected as a data string with more likelihood, the one corresponding to one of the first and second candidate sampled value strings judged closer to the sampled value string obtained from the reproduced signal in the sampled value judgment means.
As claimed in claim 5, the above-described first candidate data string may be obtained based on the first path string set in a given range in a string of the paths selected in accordance with the given rule, the given range including the path corresponding to the alternative path.
In this case, the second candidate data string may be obtained in accordance with the second path string obtained by changing the path of the first path string set as described above, the path corresponding to the alternative path selected in the alternative path selection means.
As claimed in claim 6, more specifically, the above-described first candidate data string may be obtained based on the first path string at a timing when the difference between the compared ones of the path metric values reaches the value for selecting the alternative path in a process of selecting the paths in accordance with the given rule, the first path string continuing up to the timing for a given period of time.
Further, as claimed in claim 7, in the light of providing a data reproduction apparatus having a high data detection capability and good noise immunity, in the above-described data reproduction apparatus, the above-described first candidate data generation means may generate the first candidate data string of a given length, the second candidate data generation means may generate the second candidate data string of the given length, and the data selection means may perform error detection on the first and second candidate data strings by employing a given error detection technique and select the one of the first and second candidate data strings based on a result of the error detection.
According to such a data reproduction apparatus, error detection is performed, in accordance with a given error detection technique, on the first candidate data string generated in accordance with the first path string selected in accordance with the given rule, and on the second candidate data string generated in accordance with the second path string obtained by changing to the alternative path the path corresponding to the alternative path selected based on the difference between the compared ones of the path metric values which difference is capable of representing the reliability. Based on the results of the error detection, one of the candidate data strings from which one no error is detected, for instance, is selected as data with more likelihood.
The given error detection technique is not limited to a particular one, but may be the technique of ECC (Error Correcting Code) or the technique of a parity check as claimed in claim 8.
As claimed in claim 9, in the light of preventing a decrease in accuracy of data generated based on the path string selected in accordance with the given rule, in the above-described data reproduction apparatus, the data selection means may select the first candidate data string if an error is detected from each of first and second candidate data.
As claimed in claim 10, in the above-described data reproduction apparatus, in a case where more than one alternative path is selected in the alternative path selection means and exists when the first path string corresponding to the first candidate data of the given length is selected, the second candidate data generation means may generate a plurality of second path strings by employing the alternative paths by ones, or by twos or more, and generates a plurality of second candidate data strings corresponding to the second path strings, and the data selection means may select the one of the first and second candidate data.
According to such a data reproduction apparatus, one of the first candidate data string and the second candidate data strings is selected as likely data. An increase in the number of candidate data strings subjected to the selection allows the reproduction of data having more likelihood.
As claimed in claim 11, in the light of making an operation for obtaining basic information (the string of the paths, the alternative path string, the first candidate data string) and an operation for generating likely data from the basic information independent of each other, the above-described data reproduction apparatus may include a memory unit for storing the string of the paths selected in accordance with the given rule, information on the difference between the compared ones of the path metric values, the difference being calculated in said difference calculation means, and the first candidate data string generated in the first candidate data generation means in the reproduced data determination means, wherein the alternative path selection means for selecting the alternative path based on the string of the paths and the information on the difference between the compared ones of the path metric values stored in the memory unit, the second candidate data generation means for generating the second candidate data string based on the string of the paths stored in the memory unit and the alternative path selected in the alternative path selection means, and the data selection means for selecting the one of the first candidate data stored in the memory unit and the second candidate data generated in the second candidate data generation means may operate in accordance with a synchronizing clock different from a reference clock employed for selecting the string of the paths, calculating the difference of the compared ones of the path metric values, and generating the first candidate data.
As claimed in claim 12, in the light of performing an operation for generating the likely data with higher speed, the synchronizing clock may be faster than the reference clock.